The invention relates to a method for enhancing the surface of a substrate and products produced thereby. More particularly the invention relates to a method of treating a substrate by thermally spraying a catalyst precursor material directly onto the substrate surface to increase porosity and surface area, and substrates formed thereby. The invention is particularly useful in treating a metal separator used in a micro-component reaction chamber to form a catalyst surface coating thereon.
In the prior art, material coatings typically have been applied to metallic or other surfaces for protective or restorative purposes, such as wear and corrosion resistance, thermal barrier coatings (xe2x80x9cTBCsxe2x80x9d), protection against oxidation, and dimensional restoration. Coatings made from ceramic materials in particular offer resistance against abrasive wear and thermal shock and provide electrical insulation.
The thermal spray coating process has evolved from a method for patching or repairing material surfaces to a technique enabling surface microstructure design and enhancement. As is known to those skilled in the art, thermal spraying involves any of several methods by which a coating material is placed in the path of a spray jet, heated until the material softens or melts, and propelled to the surface of a prepared substrate to form a deposit. U.S. Pat. No. 5,900,283 to Vakil et al. and U.S. Pat. No. 5,985,368 to Sangeeta et al. describe various methods of depositing a protective coating on a metal-based substrate, including thermal spraying.
Thermal spraying processes include flame spraying and plasma spraying. In each process, the material is impacted as it is deposited on the substrate and is allowed to cool and solidify to form a coating on the substrate. The deformation of the softened or molten material upon impact with the substrate, and, in some cases, the force of the impact, is sufficient for the material to mechanically bond to the surface. In thermal spraying processes, the particles sprayed, upon impact with the substrate surface mechanically lock and/or bond into the profile of the surface.
Initially, thermal spray technology focused on techniques of heating particles into a plastic state before impact to effect bonding with the surface. Methods of high velocity spraying now exist whereby extreme spray velocities are generated. Powder material is injected into a focused gas stream and imparted with enough velocity that the force of impact of particles with the substrate is sufficient to achieve adequate bonding.
Flame spraying and plasma spraying are conventional methods by which coatings are applied. Flame spraying involves the heating and projecting of the coating material through the use of an oxygen fuel flame and pressurized carrier gas jet. Coating material is melted, atomized, or softened as it is fed into the flame, and the soft or molten particulate is ejected in a directed stream through the nozzle of the spray gun toward the substrate. Like flame spraying, plasma spraying also heats and projects particles of a material toward a work surface or substrate. A plasma gun replaces the flame used in older spray systems with a stream of highly ionized inert gas, plasma, and imparts greater velocity to the powder. In the plasma spray system, an electric arc is created and a mixture of combustible gases is ignited to create a high temperature flame. The resulting plasma flame can be pushed forward in front of the gun. When powder is injected into the plasma flame near the front of the plasma spray gun nozzle, the gases expand rapidly, and the resulting velocity of the heated powder particulate propels it to the substrate. In both flame spraying and plasma spraying, large size particles or powders (typically greater than 10 (micrometers) are injected.
Coatings for protective and similar applications are formed by spraying hot powder particles onto a substrate to form coatings with thicknesses ranging from about ten up to hundreds of micrometers. The resulting coating exhibits a relatively dense and smooth, or low-profile, surface with large particle sizes. Thermally sprayed ceramic coatings may exhibit physical characteristics such as a durable, higher profile surface suitable for gripping and anti-skid applications. In applications such as sliding wear situations, in which a smoother surface is required, the coated surfaces may be further refined by grinding and polishing.
Thermally sprayed coatings have a relatively weak bond with the substrate. Internal stresses, especially in thicker coatings, and high stress wear patterns may exceed the bond between the coating and substrate and cause failure. Surface treatments that are known in the art, such as grit blasting, enhance the work surface and provide an anchor profile for the coating so that it will better adhere to the substrate. The resulting high profile coating can be polished or otherwise refined as discussed above to achieve a smooth surface in, for example, protective coating applications.
In the present invention, the thermal spraying process produces on a surface a small particle size and high surface area coating that is desirable in applications where the coating is a catalyst itself, or where the treated surface is a precursor surface for a further coating. The thermal sprayed treated substrate surface requires no further refinement.
A method for applying and forming a thermally sprayed coating on a substrate that yields a thin layer of a catalyst coating with small particle size and high surface area is the desirable object achieved by the present invention. A coating applied to a substrate using thermal spraying techniques in which the coating has mechanical stability and promotes catalytic reactions is yet another object achieved by the present invention
Catalytic material has been used as a coating on ceramic or metallic substrates in applications such as, for example, catalytic converters in automobile exhaust systems to reduce the emission of noxious gases. In the prior art, the substrate is typically coated with a catalyst by immersion in a slurry containing the catalytic material. Problems arise in that catalytic material does not adhere as well to a metallic substrate as it does to a ceramic substrate and the coating is not uniformly distributed.
U.S. Pat. No. 5,721,188 to Sung et al. discloses a thermal spray method for adhering a catalytic material to a metal substrate. The disclosed method involves thermally spraying refractory oxide particles onto a substrate for the primary purpose of attaining an undercoat having high surface roughness, and the subsequent application of a separate catalytic material to the undercoated substrate. Refractory oxide powders ranging in average particle size from 13 to 180 microns are thermally sprayed onto the substrate (See, for example, column 3, lines 27-32). Catalytic material is then applied to the undercoated substrate by immersing the substrate in a wash coat slurry containing the catalytic material. The coated substrate is then dried and calcined (Id., column 4, lines 30-35). Sung et al. recognized the need for a method to improve the adhesion between metallic substrates and catalytic materials disposed thereon (Ibid., column 1, lines 29-31) but do not propose a method for adhering a catalytic material directly to a metallic substrate.
The present invention provides a method of coating a substrate such that the coating bonds directly with the surface of the substrate. Characteristics of the thermally sprayed coating, including high porosity, high surface profile and surface area, and small particle size, are beneficially achieved in the application of the present invention. In the method of the present invention, a catalyst precursor material such as a powder having a large particle size, for example, greater than 10 micrometers, is thermally sprayed onto the substrate and forms a catalyst coating that bonds to the substrate surface. The coating is formed from decomposition products of the sprayed material having small particle size, namely in the order of less than approximately 5 microns, and more specifically in the order of less than approximately 3 microns. The substrate may in turn be coated with a further coating of a catalytic material. In a useful embodiment, the coated substrate is used as a separator between alternate fluid flows in a micro-component reaction chamber, or heat exchanger, and the coating acts as a catalyst to promote a chemical reaction in a fluid flowing over the substrate in the chamber.
In one embodiment, the present method is applied to form a catalyst coating on a separator element in a micro-component heat exchanger such as that described in United States application for U.S. Pat. No. 09/627,267 filed on Jul. 28, 2000, entitled xe2x80x9cMulti-purpose Micro-channel Micro-component,xe2x80x9d the disclosure of which is hereby incorporated by reference. The catalyst material formed in the thermal spray promotes a chemical reaction when a reagent fluid flows through the channels of the micro-component device. One or both sides of the separator may be treated. In this aspect of the invention, the coating, and the enhanced surface properties thereof, also assist in the transfer of heat between facing channels in the micro-component device.
In another embodiment, the method of the present invention comprises thermally spraying aluminum hydroxide particles onto a metal substrate. In yet another embodiment, the method may comprise thermally spraying catalyst precursor material onto a surface, or oppositely facing surfaces of a metallic substrate. In the invention, a large size powder of a precursor material is flame sprayed or plasma sprayed onto a substrate to produce a small particle size coating by the pyrolysis process, i.e., the decomposition by heat of the sprayed material.
Before treatment in accordance with the invention, the substrate surface may be enhanced by methods such as grit blasting to improve adhesion of the thermally sprayed coating to the substrate.
The invention is described more fully in the following description of the preferred embodiment considered in view of the drawings in which: